ncurses: correct check for gcc >= 5.0

The hack in question is intended to workaround seemingly bogus #line markers
in cpp output. As far as I can tell, llvm cpp doesn't do this by default, so
there's no reason to add -P.

In our /bin/sh, the main incantation should be placed in a sub-shell in
order to properly pipe the output to fgrep.

The main motivation for this change is admittedly to stop emitting the noise
about clang not being gcc in make -s buildworld

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22952